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ADAHRS Vibration and ADAHRS Mounting Brackets

NASA515

Well Known Member
When I first started my airplane's engine, and did initial ground runs, up until the present, I have had periodic verbal Caution messages, with an annunciation CAUTION: ACTV ADAHRS VIBRATION.

I posted on the Dynon Forum, and receiving no response, I called Dynon, to find out what it meant - especially the ACTV part - since that's not called out in the Installation Manual. Initially, they couldn't tell me - but later we figured it out as meaning ACTIVE - related to systems where there are two ADAHRS installed. and they recommended replacing the ADAHRS. I elected not to do so until I could track down the source of the vibration. I figured it was pointless to go through the contortions needed to crawl back into "that hole" - made even narrower now with the fuel tank installed, if I could even do it, that is, without pulling the tank - only to find that the reading was accurate and not related to the ADAHRS.

During my Phase 1 flight testing, the Caution came and went. I tried turning that Dynon Lady off, but couldn't, and have grown accustomed to her talking to me throughout most flights. She even started saying "G's", which took me a month to figure out. The vibration would sometimes trip the G-Meter, which I reset to 3.0

To gauge my vibration, I made flights with Tony T. in N577RV. I also had made multiple flights in the Vans demonstrator. My vibration - meaning mostly engine vibration - seemed more than his, but not out of line. He had done a prop dynamic balance. In any event, I went through a long process of trying to check and eliminate any vibration, including checking the engine mounts, the prop spinner run out, and doing multiple blade pitch resets. I even had Jim Scott of Aircore Aviation at Arlington do a vibration check and prop reset, including a careful setting of the blades to ensure there was no difference between them in pitch. I am also setting up a prop dynamic balance - which I figured was my last chance.

I downloaded all my data after each flight and analyzed the vertical and horizontal acceleration. Horizontal seemed OK, but there was definitely a lot of Vertical G. I did several RPM vs Vibration surveys, running from about 3500 rpm to 5200 rpm in 200 rpm increments and plotted the results to find some correlation between the two. The correlation, if any, seemed tenuous at best - the engine does seem to vibrate more around 4800 rpm (considered typical for 912s by many.) I also did similar airspeed checks and did a big Internet search. I thought I could trigger it sometimes doing stalls.

The vibration started to affect the PFD - with the horizon line and slip/skid ball jittering - sometimes very badly. Dynon sent me a new ADAHRS.

To make a long story short, I am now on my third ADAHRS box. Changing them is NO FUN. The third one I installed with thick rubber spacers between the ADAHRS feet and the Vans ADAHRS brackets - still no joy. I still have the vibration. I've had sit down meetings with Dynon about this.

A few days ago, as I was standing alongside my airplane, I pushed the area where the ADAHRS mounting brackets are located - I was amazed. The crown skin in that area was VERY flexible - almost like a trampoline. If you put your thumb on one row of support bracket rivets, and your other fingers on the other rivet line and rock back and forth, you will find - or at least - I found - there was tremendous flexibility.

I thought I had found my problem.

Doing the same at the F-1208 frame shows it to be very rigid -as to be expected. The crown skins between the frames are VERY flexible. Using the template, I re-confirmed the brackets were correctly located.

On Thursday, I flew my airplane into the Arlington Airshow. At Van's request, I parked my airplane on their show line next to their RV-7A and RV-14A demo airplanes - between the Vans and Dynon display tents. Steve has been my Dynon Tech Support contact - he was manning their tent. I showed him my findings - he was amazed and felt sure that was the source of my problem. Joe Blank was manning the Vans tent - same reaction. The second Vans guy (whose name eludes me) said they had been having the same problem on at least one factory built S-LSA airplane during flight testing.

Everyone felt this area would be subjecting the ADAHRS to a high level of vibration susceptibility - either due to engine/airframe induced vibrations, or simply due to aerodynamic forces at different airspeeds and attitudes, as the airstream (perhaps) burbled off the aft canopy/crown area (especially during high nose attitude maneuvers.)

Everyone agreed I needed to beef up the area, by stiffening the crown skin in the transverse direction (athwartship is the Boeing term). Various schemes were discussed - from using a part of a new F-1208 frame, fabricating from scratch stiffeners, and gluing or riveting them to the crown skin, to bonding lead/rubber pads (as I have used on boats) to the inside skin. Even attaching an external transverse doubler. I've also thought of duct-taping some sort of doubler to the outside and see if that eliminates the vibration problem.

I recall an assortment of other postings here that described verbal "G's" messages, and jittering horizons and slip/skid balls. All may be related and a result of this installation configuration.

I plan to call Vans first thing Monday morning and get their views and advice, before I engineer my own solution, and will keep the crew here advised.

Bob Bogash
N737G
Now at 75 Flights
 
Wow Bob, thanks. Like hundreds of other 12 owners, I rushed to the garage and pushed around on that area, you are right, it is spongy beyond belief! I will be paying attention to your fix, while I still have not put my tank or rear bulkhead in. Should be an easy fix for sure.
 
You might want to consider the use of 3M Double Sided Tape. I think it comes in several strength ranges. I have addressed several oil cannings by using this tape and some light gauge aluminum bent into angle. It works great and is very strong. I believe it is the same product used to bond the Kruger Sun Shields to the canopy.
 
I haven't started the tailcone yet so I'm not sure where the ADAHRS actually mounts. Do you have a picture? It's between the F-1208 and...

Thanks,

Bob
 
This photo might help. You must ignore the reflection however. The strength in the center is nice, but you can see how it would "rock" on the mounting.
i6fhvr.jpg
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I haven't started the tailcone yet so I'm not sure where the ADAHRS actually mounts. Do you have a picture? It's between the F-1208 and...

Thanks,

Bob
 
It's on the top centerline between the Aft Cargo Bulkhead and the F-1208 frame.

Bob Bogash
N737G
 
Good catch!

Thru actual flight experience, the Van's RV-12 is getting better and better. I appreciate all of the 'pioneering' of those of you who have completed and are flying 12s. Wonder how Van's will fix this? Should be pretty simple, I hope.

Thanks to all of you who are flying 12s - I hope to start construction soon!
 
That double sided attachment tape is really something - from Walmart Aviation, it is less than 1/8 inch thick. I used it to attach my rudder wedge, thinking "If this wedge happens to need shortening, I'll just peel it off and eventually final-mount with RTV." Well, the wedge was perfect, so I started to (try to) peel it off for the RTV. Hmmmm. You know, I think it is perfect as it is! I have no doubt it would easily solve oil canning if attached to aluminum angle.
 
Talked to Gus at Vans first thing this morning. They've heard of a few, but don't think it's endemic. The S-LSA problem was solved by re-pitching the prop and ensuring equal angles between blades (which I've already done - 3X.)

His advice - add a stiffener, with glue, pro-seal, tape, rivets - whatever suits my fancy, and report back. It could be transverse - or even longitudinal, tying the brackets to the frame behind it. Funny - Dynon's very first suggestion was to beef up the brackets.......

I plan to get the prop dynamically balanced first to eliminate that avenue once and for all. If that doesn't work, I may scuba dive again into "that hole" and conjure up some sort of stiffening arrangement.

Meanwhile - back at the ranch - since we made this "Ah-Ha" discovery and amazed the Vans and Dynon folks at the Arlington Show - the system has felt threatened and to defend itself has gone cold-turkey on these warnings. Since then, I've flown KAWO - KPWT - KHQM - KPWT -- about 3-4 hours with the box performing just like good little boxes are supposed to perform. Not a peep out of it.

To be continued......

Bob Bogash
N737G
 
ADHRS Vibe

I have seen the message at higher power/airspeed settings. Good to know others have same issue and it is going to be addressed.
 
I have seen the message at higher power/airspeed settings. Good to know others have same issue and it is going to be addressed.

From the suggestions they made to Bob, it seems that Vans don't see it as much of a problem, and will leave it up to individual owners to do something if they think they need to.
 
Probably depends upon how many experience difficulties. Recall the whole MLG beef-up story, that began with quiet whispering, private messaging, some finger pointing, disbelief, then acceptance and ultimate resolution.

There is probably a fair amount of variability in the sponginess of the structure in that area depending upon builder assembly technique together with variability in individual ADAHRS units and their degree of robustness relative to vibration.

These acceleration sensors, BTW, are off-the-shelf units that are normally used to trigger car air bags. As such, they are binary devices that are off - then on - in a collision. Dynon is using them as analog devices to sense a range of accelerations which are reported to the system software.

Skyview uses these data inputs in quite a wide variety of functionalities - beyond just displaying them to the pilot on the G-Meter, or triggering a Caution. The Autopilot in particular is a heavy user, and excessive G readings first truncate functionality, and then make the system essentially unusable.

Also, recall - if you review the Dynon manuals - annunciation of this Caution only began a few firmware revisions ago. Before that, the user was unaware of this problem. If he experienced an assortment of difficulties, Dynon would be the ones who could pinpoint the cause - presumably by sending them a down-loaded Diagnostic file.

Regardless, in my view, the highly flexible structure in this area is very much the opposite of the rigid mounting demanded of an ADAHRS installation. Even if one experiences no Vibration messages (and - as mentioned above - my system seems to have gone 98% cold turkey since I pointed my finger at it) - higher levels of vibration - while still below the Caution trigger - will degrade some system functionality. Therefore, I plan to go ahead and install some sort of stiffening beef-up - I just don't know what at present.

Bob Bogash
N737G
 
Probably depends upon how many experience difficulties. Recall the whole MLG beef-up story, that began with quiet whispering, private messaging, some finger pointing, disbelief, then acceptance and ultimate resolution.

There is probably a fair amount of variability in the sponginess of the structure in that area depending upon builder assembly technique together with variability in individual ADAHRS units and their degree of robustness relative to vibration.

These acceleration sensors, BTW, are off-the-shelf units that are normally used to trigger car air bags. As such, they are binary devices that are off - then on - in a collision. Dynon is using them as analog devices to sense a range of accelerations which are reported to the system software.

Skyview uses these data inputs in quite a wide variety of functionalities - beyond just displaying them to the pilot on the G-Meter, or triggering a Caution. The Autopilot in particular is a heavy user, and excessive G readings first truncate functionality, and then make the system essentially unusable.

Also, recall - if you review the Dynon manuals - annunciation of this Caution only began a few firmware revisions ago. Before that, the user was unaware of this problem. If he experienced an assortment of difficulties, Dynon would be the ones who could pinpoint the cause - presumably by sending them a down-loaded Diagnostic file.

Regardless, in my view, the highly flexible structure in this area is very much the opposite of the rigid mounting demanded of an ADAHRS installation. Even if one experiences no Vibration messages (and - as mentioned above - my system seems to have gone 98% cold turkey since I pointed my finger at it) - higher levels of vibration - while still below the Caution trigger - will degrade some system functionality. Therefore, I plan to go ahead and install some sort of stiffening beef-up - I just don't know what at present.

Bob Bogash
N737G

Bob,

What about using a second F-1208 mounted mid-way between the F-1208 and fuse connection? Some work to the 1208 (both sides) where the horizontal stiffeners run, but the curvature looks to be same. Would need a connection piece top and bottom.
 
Bob,

What about using a second F-1208 mounted mid-way between the F-1208 and fuse connection? Some work to the 1208 (both sides) where the horizontal stiffeners run, but the curvature looks to be same. Would need a connection piece top and bottom.

That was my first thought. There would be contour issues due to the different cross-section, but since I would propose just using a small segment of the frame, it might be made to conform.

I don't know what one of those frames costs. I think I could probably bend up something that would work for a lot less money.

Need to ponder some more and I enjoy the poster's ideas.

Bob Bogash
N737G
 
One solution ...

Thought I'd see if I could stiffen up the fuselage around the ADAHRS mounting and after a bit of head scratching came up with this:

9332255257_ac15715192_z.jpg


I didn't want to attach anything directly to the ADAHRS bracket because it's too difficult to work in that confined space, and I also didn't want to have to touch those pesky little bolts again to remove the unit.
Seems to work pretty well. The whole area is very much stiffer now, and I think the ADAHRS should be solid enough. All parts are made from 0.025" because that's what I had available, but thinner material could be used for the two ribs. I wouldn't go any thinner for the centre joining plate. The plate is important, because the ribs need to be connected together with two rivets each side to work properly. I made a second set of ribs to go behind the ADAHRS unit, but didn't install them because the first set seemed to do the job. I don't have my turtle deck riveted in yet, so I was able to install the new parts without climbing into the back.

The ribs are 5/8" deep at each end, top flange is 1/2" wide, bottom flange 3/8". Rivet spacing is 1-3/16" with the first rivet 1" from top line of fuselage rivets. Centre plate is 1/2" deep under the J stiffener, and is about 6" long. Drill the two rivet holes for the plate in each of the ribs before installation, and then match drill the plate later. I had already put the ribs in before drilling the holes, which resulted in me not getting the rivets quite where I wanted them because it was a bit awkward to reach in and position the drill accurately, but they will have to do now.
 
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Those look great! All cut on a bandsaw and just hand-bent? Those look factory produced! Nice work / report.
 
Bob - that is a first rate job!!! Just what I had in mind. You Aussies are OK.

For the rest of you guys - you didn't hear it here, and you didn't hear it from me, but..........

Vans engineering advised a few days ago they had designed a stiffening kit and were prototyping the hardware. It will be available for new-builds and retrofit. It will be released in a few weeks.

They didn't want me to say anything because they didn't want all you eager-beavers out there to start calling the support line and the parts ordering desk driving them nuts trying to get the kit. So don't violate me and my worm hole into Vans engineering or they'll cut me off.

I'm only reporting this now so a bunch of guys don't run around designing their own fixes, when Vans is working on a kosher fix.

Bob Bogash
N737G
 
Thanks for the update, Bob. Your secret info is safe here. Hey, it's the Internet. Nobody will find out you shared the news with our private fraternity here ;)

In all seriousness, thanks. You saved me a lot of fabrication in the coming days.
 
Thanks Bob. Good to know Vans are working on a fix. Guess I should have followed my usual engineering philosophy at work, "do nothing, and stay ahead". Sometimes it pays to wait. ;)
 
That's life, Scott. Better to skate on the wrong side than have a hundred guys devising and installing their own fixes over the next month or so.

A judgement call that I deem well worth it.

Bob Bogash
N737G
 
The email answer I got from Van's an hour ago was they know nothing about an ADAHDS mount stiffening kit. I even mentioned the rumor posted earlier in this thread.

I am trying to order the secret parts for the new ADSB tunnel cable brackets. Not sure why Van's does not put the mod parts in the List or options.

John Salak
RV12 #120116
 
John,
I also checked with Van's support late last week and got the same "no knowledge" response. Perhaps they are not fully clued in on what's going on in the Engineering group. That would not be all that unusual. Advised by more knowledgeable members of this forum, I'm not going to sweat it for now. The fix, whenever it comes out, should be an easy retrofit for us in the building process. I'm going to wrap up the tail cone and move on.
 
Vibration

I had the same problem and could not fix it. Eventually I put on a 3 blade prop and that stopped it. When I talked to Dynon about it they said it was probably coming from the D180 and vibration. Maybe not. the three blade prop is very smooth but not as efficient as the 2 blade.

Richard
120002
 
Field Problem Report?

I recall a short while back when the loose landing gear / fuse skin wrinkle issue surfaced, it was being discussed at some length here on VAF, but the chaps at Van's seemed to be the last to know about it. Wasn't it Scott M who piped up here and asked those owners to submit a report to the factory? I just looked on the factory site for some kind of report form but couldn't find one.

Has anyone (no, make that everyone) who's experiencing the "G" alarm reported it to Van's via whatever official channel they have?
 
Fix Kit?

...you didn't hear it from me, but..........
Vans engineering advised a few days ago they had designed a stiffening kit and were prototyping the hardware. It will be available for new-builds and retrofit. It will be released in a few weeks.
They didn't want me to say anything because they didn't want all you eager-beavers out there to start calling the support line and the parts ordering desk...

Since this was dated 7/31, am wondering what constitutes "a few weeks." Ten so far. Don't wish to be prematurely pestering the Mothership, so am posting here instead. Any scoop on the jungle telegraph?
 
I'm with you 110% and am glad this subject has been re-opened.

Dynon and Vans personnel witnessed the ADAHRS area movement personally at the Arlington Airshow - where my airplane was parked between their two display tents. The Dynon rep took a video of the movement (similar to the one DonfromTx made) and sent it to Vans engineering.

Vans engineering told Dynon they would create a fix and release it after Oshkosh. So far - nothing.

A week or two back, I was standing behind the wing and I had my Skyview powered up with external power. I could see the screen clearly, and had the PFD display on. As I rocked the crown skin where the ADAHRS brackets are located, the PFD horizon rocked back and forth about 10-15 degrees right and left. I could also get the display to jitter and the slip/skid ball to move back and forth - all things I had seen in flight.

This is definitely a problem - I wonder whether it doesn't even impact the longevity of the ADAHRS unit itself to be seeing this vibration.

Since Vans either no longer thinks this is "a problem" or is taking their sweet time fixing it, I'm seriously considering designing and installing my own fix - similar to Bob down in Perth. That would be a shame - if a formal fix is forthcoming.

I believe this is a serious little chink in Vans armor - lack of communication. It would be simple, IMHO, to release a notice saying they are aware of the problem, are developing a solution, and all builders should refrain from coming up with their own fix pending release.

Instead, what is likely coming is another one of Van's "Stealth fixes." You know the kind that start with "Hey, I'm building kit #825 and just received my fuse kit, and there are these stiffening brackets for the ADAHRS mount......."

Bob Bogash
N737G
 
Laziness abounds around my house! I can figure out some way to fix it, but like the rest am waiting for Vans (or Bob) to engineer a good fix to save my valuable engineering time for other stuff:)
 
I'm with you 110% and am glad this subject has been re-opened...Vans engineering told Dynon they would create a fix and release it after Oshkosh. So far - nothing...Since Vans either no longer thinks this is "a problem" or is taking their sweet time fixing it, I'm seriously considering designing and installing my own fix...I believe this is a serious little chink in Vans armor - lack of communication...what is likely coming is another one of Van's "Stealth fixes."

Bob, my inquiry about the subject was meant only to find out if anyone knew of any further development. If it's all the same to you I?d rather not be a participant in criticizing Van?s. I realize quite well that you are flying and I?m far from it yet, therefore there is more riding on it where you are concerned than mere builder?s convenience. The word I get here is that Van?s is working nights and weekends to get their backlog caught up. I have no harsh thoughts toward them even though I am wishing the fixit kit were done so I can have that issue behind me before it?s time to mate the fuse and tailcone. I am building E-LSA so have no option to fab my own solution.

You may be with me 110% as you stated, but may I state most respectfully that it?s not entirely reciprocal. Late or not, what I know about them, as do you, is that they are very smart, very competent, and they care very much.

I also would like to thank you for your many contributions on this forum. Some of the exchanges have been, ah, spirited but were always interesting and informative. I have gained a lot from them, for which I am in your debt.

BTW, it is still technically "after Oshkosh." :D
 
Bob, my inquiry about the subject was meant only to find out if anyone knew of any further development. If it's all the same to you I?d rather not be a participant in criticizing Van?s. I realize quite well that you are flying and I?m far from it yet, therefore there is more riding on it where you are concerned than mere builder?s convenience. The word I get here is that Van?s is working nights and weekends to get their backlog caught up.

OK, then, John - no 110% - let's just say we agree to disagree.

Getting into that aft fuselage area is NO fun! There are threads on here just devoted to that subject. After installation and close-up, I was mightily relieved to be out of there.

But wait! After first flight, I've had to go back in there at least six more times, to replace the ADAHRS three times. I'm an old guy who could stand to lose a few pounds and that was NO FUN! I also have an artificial hip that has a flight envelope all its own and I must be constantly aware of its angular geometry, lest I pop the ball out of its socket - in which case I would be totally immobilized and necessitating a 911 call to be rescued. Hopefully, they wouldn't use the "Jaws of Life" to cut my wretched body out of the back end. I do, however, always make sure my cell phone is close at hand when I'm back there.

But wait! If you've built as an E-LSA - per the Plans - then to get into the aft fuselage, you need to remove the aft bulkhead. And to remove the aft bulkhead, you need to remove the fuel tank. And to remove the fuel tank, you have to drain the fuel system.

John, I have at least a couple more trips back there in my future - to install Vans ADAHRS stiffening fix - or if they go cold turkey on this - then to install one of my own design. I'm not looking forward to those further adventures. Not one bit.

As far as Vans "working nights and weekends to get their backlog caught up", well, all I can say is, Vans is a business enterprise, and from all appearances, quite a successful one. As their model line-up grows and their population of sold kits and in-service airplanes increases, maybe they need to add staff to accommodate the increased demand for engineering resources.

Bob Bogash
N737G
 
ADAHRS Doubler Plate

I attempted to fab a stiffener bracket similar to the one RGMWA made, except mine was one piece. After installation I was not real happy with the result.
3

Although it did reduce the flexing a bit, I decided to try a doubler plate to fix the problem.
3

The doubler is a single piece of .032 that is tapered to match the decreasing width between the J stiffeners. The curve was made by fluting the 90 deg flanges to match the skin curve perfectly. The J stiffener has a 1/2" wide .032 doubler between the skin and stiffener to match the bracket thickness. The ADAHRS bracket was riveted thru the skin and doubler.
3

This is a comparison of the two bracket approaches I tried. Although is weighs 2 oz more, the plate doubler design has eliminated any flexing in the ADAHRS mounting area. Now that I have finished, I am sure Van's will release their version any day.

John Salak
RV-12 #120116
 
John,

could you repost, please? Your pictures didn't show up and I'm very interested in your solution.

Thanks,
-dbh
 
John, the position of the rib is important. Mine is located just 7/8" in front of the most forward rivet holding the ADAHRS bracket, and is actually aft of the pressure fittings. It works well in that location and provides more than adequate stiffness. However there's always more than one way to skin a cat. :)
 
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I also considered a two piece design that incorporated the ADAHRS brackets as part of the sturcture. The one I ended up using is similar to the doublers I added for the COM2, GPS, and ADS-B antennas. You can see the COM2 antenna doubler in the photos aft of the ADAHRS bracket (along with two additional J channels to fix a similar flexing problem).

John Salak
RV-12 #120116
 
Vans or not????

The exchange Bob Bogash/Scott in this thread 7/21/2013 inferred that Vans were looking at this. It would be nice to know definitively if they are or not. It's been very quiet since. Any update Scott??
 
I communicated with Van's engineering today, and they advise they are close to having and releasing a solution. Those working on homebrew designs should stand down.

Bob Bogash
N737G
 
I put mine in upside down by mistake, could not understand why the Skyview seemed to have brown sky and blue earth!
 
I communicated with Van's engineering today, and they advise they are close to having and releasing a solution. Those working on homebrew designs should stand down.

Bob Bogash
N737G

Bump......Scott

Any news (getting ready to attach turtle back skins) and would love to be able to do work before attaching them.
 
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